Introduction
The traditional plank, or prone bridge, is a traditional bodyweight exercise designed to increase core muscular strength, endurance, and stability. The term “core” has been previously described as the musculature of the pelvis and trunk that are responsible for the stability of the spinal column (2,23). Core musculature is important for resisting spinal perturbations and transferring power across the extremities during physical activity (2,23). Increasing core muscular strength can significantly reduce the incidence of lower-back injuries, increase athletic performance and trunk stability, which makes exercises designed for the abdominal wall ideal for rehabilitation, as well as conditioning programs (5,9,11,18,22).
Recent research has shown a shift in health and fitness screenings from the traditional sit-up and crunch to a timed plank to avoid lower-back injuries and increase relevance of the test to activities of daily living (14,17). Traditional sit-ups place increasing compression forces on the lower lumbar region, which may increase the likelihood of an initial or recurrence of an existing injury (15), and therefore should be avoided. McGill (15) states that individuals exposed to repeated spinal flexion (e.g., sit-ups) may also surpass lumbar compression forces leading to various degenerative spinal column injuries (e.g., herniations). Therefore, more rehabilitation and strength and conditioning programs are opting to increase core musculature strength and endurance through plank utilization.
One of the most common methods of increasing the stabilization and balance challenge to traditional exercises is the Swiss ball (5,11–13). However, multiple studies have examined this instability device with mixed conclusions. Two studies performed by Marshall and Murphy (12,13) demonstrated significantly greater activation of the rectus abdominis (RA) during push-ups and double-leg lowering when performed on a Swiss ball as compared with a stable surface. However, the same study provided no significant difference in core muscular activation when squats were performed with and without the assistance of the instability device.
Another type of instability training, referred to as suspension training (ST), incorporates the use of hanging straps and handles. This form of training, which mimics Olympic gymnastic rings and has recently gained in popularity, has been shown to provide an equal or increased challenge for traditional bodyweight exercises (1,19–21). Beach et al. (1) and Snarr et al. (21) demonstrated greater activation of the trunk musculature (i.e., RA and external obliques [EOs]) during push-ups when performed on a suspension device. Although, when a pulling movement (i.e., inverted row) on the suspension device was examined, there was no difference in the upper back and shoulder musculature (19). However, activation of the biceps brachii during the suspended inverted row was significantly less when compared with the stable version (19). A recent longitudinal study using a suspension device was also performed by Janot et al. (8), which showed improvements in abdominal flexion, back extension, and side-bridge tests after 7 weeks of ST.
Although both instability devices have shown greater activation of musculature during traditional movements, the authors are only aware of 1 study comparing these 2 devices. Schoffstall et al. (18) cross-examined suspension devices against other instability devices, such as the Swiss ball during a prone V-up exercise. Results of the study showed no difference in trunk musculature (RA or EO) activation between either instability devices.
With such an influx of these commercial products, only a small handful of data have supported these claims of increased core muscular activation while performing traditional exercises on labile surfaces. Therefore, the purpose of this investigation was to compare the EMG activity of the RA, EOs, and lumbosacral erector spinae (LSES) while performing multiple variations of the plank with and without instability devices (i.e., Swiss ball and TRX Suspension Trainer). The authors hypothesized that the unstable surfaces would produce a significantly higher activation of the selected musculature during the plank.
Methods
Experimental Approach to the Problem
With an increasing trend in core training using instability devices, published scientific research is necessary to validate these methods. There are limited scientific data comparing core musculature activation with suspension devices vs. other common instability equipment. Therefore, the purpose of this investigation was to determine the EMG extent (i.e., the differences in raw muscular activity and percentage of a maximum voluntary contraction [MVC] between a traditional plank and instability devices) of the RA, EO, and erector spinae. The complete details of this investigation are described in the following sections.
Subjects
All subjects were recruited through flyers and word of mouth. Twelve subjects (6 men and 6 women) volunteered for this investigation. All descriptive statistics for the participants are provided in Table 1. All subjects had self-reported physical activity levels of moderate to advanced, with at least 6 months of prior resistance training experience. A health history and medical questionnaire were completed by all participants before testing to ensure that all subjects who were involved in this study were free from cardiorespiratory, musculoskeletal, and neurological disorders. Subjects with any prior injuries that would otherwise affect muscular activation were excluded from data collection. All subjects signed informed consent documents, as well as completed Health History Questionnaires prior to participation in the current study. This investigation was approved by the University's Institutional Review Board.
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Table 1: Descriptive statistics of the study participants.
Procedures
Electromyography
All EMG values were collected using a BIOPAC MP150 BioNomadix Wireless Physiology Monitoring System with a sampling rate of 1.000 kHz. Data were analyzed using Acqknowledge 4.2 software (BIOPAC System, Inc., Goleta, CA, USA). Before placing electrodes (Biopac EL504 disposable Ag-AgCl), participant skin sites were prepped for application through shaving, exfoliation, and alcohol cleansing to reduce impedance from dead surface tissue and oils. The electrode placement chosen for this investigation was consistent with Cram and Kasman (4).
Electrodes for the RA were placed 2 cm to the right of the umbilicus and 3 cm apart (vertically) directly over the RA muscle fibers. The EO electrodes were placed 15 cm lateral to the umbilicus, and halfway between the iliac crest and the bottom of the ribs at a slightly oblique angle (e.g., approximately 25°). The erector spinae electrodes were placed 2 cm parallel from the L-3 vertebrae, approximately 2 cm apart. A ground surface electrode was placed directly over the right anterior superior iliac spine.
Exercise Trials
All EMG data were collected during 1 testing session for each subject during which they performed multiple variations of the plank along with MVCs. Once all electrodes were placed, MVCs were measured to normalize all EMG signals. The following methods were used to collect MVCs and were consistent with Konrad (10).
- Rectus abdominis: Subjects assumed a supine position on a mat with the knees flexed to 90° with the arms crossed over the chest. Subjects then attempted to perform a sit-up while the investigator provided a matched resistance to elicit an isometric contraction.
- External oblique: Subjects assumed a side-lying position with hips and legs fixated to an athletic table. Next, the individual attempted a lateral spinal flexion against a matched resistance.
- Lumbosacral erector spinae: A prone lying position on an athletic training table was used to collect MVC data for the LSES. The subjects' legs and upper torso were held in place by researchers while the individual attempted to perform a standard back extension movement against a matched resistance.
Once all EMG data were normalized, subjects performed multiple variations of the plank with and without an instability device. The exercises were performed in a randomized order to prevent data fatigue error. All subjects were instructed about the proper technique by a Certified Strength and Conditioning Specialist (NSCA-CSCS). If any subject was not able to maintain proper form as instructed, then all data were omitted from the analysis process. Each variation of the plank was held for a 5-second isometric contraction and was repeated for 2 complete repetitions. During data collection, each subject was allowed a 3-minute rest between exercises to prevent fatigue of the trunk musculature. The proper technique of each exercise used in this study is as follows:
- Regular plank (REG): To perform the traditional plank, subjects were instructed to assume a prone plank position on an exercise mat with their elbows flexed to 90° with only the forearms and toes in contact with the ground. Subjects were instructed to maintain a rigid torso, neutral head and spine, and extended leg position throughout the exercise.
- Plank with elbows on Swiss ball (EB): Subjects were to assume the above-stated position, but with the forearms placed on the Swiss ball and toes on the ground resulting in an inclined plank position. A rigid torso, neutral spine, and extended leg position was to be maintained throughout.
- Plank with feet on Swiss ball (FB): During this variation, subjects were instructed to place the feet on the Swiss ball while placing the elbows and forearms on the ground producing a declined plank position.
- Plank with elbows in TRX (ET): Before performing this exercise, a suspension device (TRX Suspension Trainer, Fitness Anywhere, LLC, San Francisco, CA, USA) was secured overhead to a Smith machine. The handles were placed approximately 6–8 in above the ground to create a horizontal positioning of the subject. The subjects then assumed a plank position directly beneath the Smith machine with the feet placed together on the ground, while the forearms were placed inside the straps of the suspension device.
- Plank with feet in TRX (FT): This variation called for the subjects to assume a plank position directly beneath the Smith machine and TRX device, with the feet placed inside the foot straps and forearms resting on the ground.
Statistical Analyses
Data analysis was performed using SPSS/PASW Statistics version 18.0 (Somers, NY, USA). Mean and SD values were calculated for each variable (RA, EO, and LSES). Repeated-measure analysis of variance was used to determine if the raw (mV) and normalized (%MVC) values for the RA, EO, and LSES were significantly different across the 5 exercises. A priori statistical significance was set to a value of p ≤ 0.05. The magnitude of the differences between the instability devices and traditional plank (i.e., REG) was determined with Cohen's d procedure (3).
Results
All subjects completed the exercise trials successfully, and all data were included in the statistical analysis process. Mean (±SD) for raw and %MVC values of the selected superficial musculature across the plank variations are shown in Table 2 (raw) and Table 3 (%MVC).
Table 2: Comparison of the raw electromyographic values (mV) of the selected musculature between the different exercises.*
Table 3: Comparison of the normalized electromyographic values (%MVC) of the selected musculature between the different exercises.*
Rectus Abdominis
There were significant differences in the raw and %MVC EMG activity of the RA across the 5 exercises. The ET provided the highest mean raw and %MVC muscular activation and was significantly different than EB, FT, and REG (raw) and REG, EB, FB, and FT (%MVC), whereas the traditional plank (REG) provided the lowest activation (raw and %MVC) and was significantly lower (p ≤ 0.05) when compared with the remaining exercises (Figures 1 and 2). Compared with REG, the Cohen's d statistic determined that the effect size was 0.50 for EB, 0.63 for FB, 0.79 for ET, and 0.33 for FT.
Figure 1: Comparison of electromyographic activity (mV) of the rectus abdominis during planks performed with and without instability devices. *Significantly lower compared with EB, FB, ET, and FT (p ≤ 0.05). †Significantly lower compared with ET (p ≤ 0.05). ‡Significantly lower compared with ET and FB (p ≤ 0.05). EB = elbows on ball plank; FB = feet on ball plank; ET = elbows in suspension device plank; FT = feet in suspension device plank.
Figure 2: Comparison of normalized electromyographic activity (%MVC) of the rectus abdominis during planks performed with and without instability devices. *Significantly lower compared with EB, FB, ET, and FT (p ≤ 0.05). †Significantly lower compared with ET (p ≤ 0.05). ‡Significantly lower compared with ET and FB (p ≤ 0.05). EB = elbows on ball plank; FB = feet on ball plank; ET = elbows in suspension device plank; FT = feet in suspension device plank.
External Oblique
In terms of the EO, the exercise that elicited the highest EMG activation (raw and %MVC) was the FB, whereas the REG plank demonstrated the lowest. The REG plank was significantly lower (p ≤ 0.05) when compared with planks performed on instability devices. Although FB provided the highest activation levels, it was not significantly different from ET. However, both FB and ET (raw) were significantly different (p ≤ 0.05) than FT, EB, and REG. Only FB (%MVC) was significantly higher than ET, EB, and REG (Figures 3 and 4). Compared with REG, Cohen's d determined that the effect size was 0.51 for EB, 1.04 for FB, 0.99 for ET, and 0.53 for FT.
Figure 3: Comparison of electromyographic activity (mV) of the external oblique during planks performed with and without instability devices. *Significantly lower compared with EB, FB, ET, and FT (p ≤ 0.05). ‡Significantly lower compared with ET and FB (p ≤ 0.05). EB = elbows on ball plank; FB = feet on ball plank; ET = elbows in suspension device plank; FT = feet in suspension device plank.
Figure 4: Comparison of normalized electromyographic activity (%MVC) of the external oblique during planks performed with and without instability devices. *Significantly lower compared with EB, FB, ET, and FT (p ≤ 0.05). ∫Significantly lower compared with FB (p ≤ 0.05). EB = elbows on ball plank; FB = feet on ball plank; ET = elbows in suspension device plank; FT = feet in suspension device plank.
Erector Spinae
The exercises that produced the highest raw EMG data were both the EB and ET, with the highest %MVC in the ET plank. The exercise that elicited the lowest activation was REG, which was significantly different (p ≤ 0.05) than the remaining exercises. The only remaining exercise that had a significant difference, in both raw and %MVC activation, was FT, which was significantly lower (p ≤ 0.05) when compared with ET and EB (Figures 5 and 6). Compared with traditional plank, Cohen's d determined that the effect size was 1.01 for EB, 0.11 for FB, 1.11 for ET, and 0.59 for FT.
Figure 5: Comparison of electromyographic activity (mV) of the lumbosacral erector spinae during planks performed with and without instability devices. *Significantly lower compared with EB, FB, ET, and FT (
p ≤ 0.05).
Significantly lower compared with ET and EB (
p ≤ 0.05). EB = elbows on ball plank; FB = feet on ball plank; ET = elbows in suspension device plank; FT = feet in suspension device plank.
Comparison of normalized electromyographic activity (%MVC) of the lumbosacral erector spinae during planks performed with and without instability devices. *Significantly lower compared with EB, FB, ET, and FT (
p ≤ 0.05).
Significantly lower compared with ET and EB (
p ≤ 0.05). EB = elbows on ball plank; FB = feet on ball plank; ET = elbows in suspension device plank; FT = feet in suspension device plank.
Discussion
Although recent trends in fitness have incorporated multiple types of instability devices (e.g., Swiss balls, suspension devices, etc.) in exercise programming, there has been limited research as to their effectiveness. Commercial claims report increases in core musculature while performing traditional exercises (e.g., planks) on these products. Currently, the authors are aware of only 1 such study that provides a cross-comparison of multiple instability devices vs. traditional exercises. Therefore, the purpose of this investigation was to compare the EMG activity of the RA, EO, and LSES between multiple variations of the plank with and without instability devices. The most important finding of this study was the significantly greater EMG output of the analyzed musculature when planks were performed on the labile surfaces (i.e., Swiss ball and suspension device) compared with the traditional method.
The results of this study are consistent with previous research indicating that instability devices (i.e., ST and Swiss ball) elicit greater EMG activity of the RA, EO, and LSES compared with traditional exercises designed to target the abdominal wall (1,2,5,11,12,21,24). For example, Lehman et al. (11) showed greater RA and EO activity when planks were performed on a stability ball compared with the traditional method. Two other studies also demonstrated similar results (i.e., increased RA and EO activity) during curl-ups on a Swiss ball vs. the traditional stable method on the floor (5,24). Behm et al. (2) also had subjects perform multiple trunk-strengthening exercises (e.g., side bridges, supermans, prone bridges, etc.) with labile surfaces, which showed that unstable surfaces increase EMG in the abdominal wall and LSES region as well.
A similar study showed increased RA, EO, and LSES when participants performed supine bridges with a Swiss ball as compared to without (7). These results suggest that with a decrease in surface contact area between the participants and the unstable surface, the greater the increase in EMG activity in the trunk musculature. In other words, as the feet or forearms are placed in or upon the instability device, it seems to provide an increased challenge to spinal stability (13). The trunk musculature is vital for ensuring integrity of the vertebral column and resisting excessive rotations during isometric contractions. Therefore, the increased EMG activity of the RA, EO, and LSES when planks are performed with instability devices may also be caused by a greater demand for core stabilization to prevent or resist spinal perturbations during the exercise (13).
With most dynamic and isometric movements where the body is in a prone plank-like position while performing exercises, there is a greater need for core activation when compared with traditional supine spinal flexion movements (e.g., crunches). Two studies have shown similar EMG patterns of the core musculature when comparing push-ups performed on a ST device vs. traditional crunches (1,21). It seems that there is greater activity within core musculature (RA, EO, and LSES) when exercises involving plank-like positions (e.g., push-ups) are performed on unstable surfaces, possibly because of the maintenance of proper spine positioning (1,21). This greater increase in EMG activity may also be contributed to the distance of the center of mass from the labile surface and decreased surface contact area. Marshall and Murphy (13) additionally suggest that the further the center of mass from the labile surface, along with a decrease in surface contact area, a greater increase in EMG activity may be experienced.
The significantly greater increases in LSES muscle activity can possibly be contributed to the higher amount of activation required by the posterior trunk musculature to maintain the plank position, along with co-contractions of the anterior trunk (i.e., “bracing”) to prevent trunk extension or rotation (11). However, with an increase in LSES EMG activity, an increase in spinal loading and compression may occur. Therefore, the plank variations performed with labile surfaces may be considered advanced maneuvers and be avoided by those with a history of injury or weaknesses in the lower-back musculature (16).
Another interesting finding during this study was the significant differences between instability devices. Although the ET provided the highest amount of RA activation, the authors would assume that the same positioning on the Swiss ball would activate the RA in a similar pattern. However, the FB provided a higher activation of the RA than the EB, but the magnitude of the difference was considered small (Cohen's d = 0.18) (3). This small difference in muscular activation patterns can be explained by the increased distance of the center of mass from the labile surface during the EB and FB. A study performed by Marshall and Murphy (12), demonstrated similar results such that the RA was activated to a significantly greater extent when a Swiss ball push-up was held in the top position as compared with an isometric hold at the bottom position.
Furthermore, the EMG patterns may differ between the suspension device and Swiss ball because of stabilization and degrees of freedom within the 2 devices. While performing planks on instability devices, there must be an increase in synergists and stabilization musculature to keep the body aligned as compared with a stable surface. However, differences exist between the designs of these 2 devices. For instance, the Swiss ball operates as a single unit, whereas the suspension device is comprised of 2 independent handles capable of moving in different directions. While performing the EB, the forearms are placed on the Swiss ball with the primary stabilization joint for the upper body (shoulder) avoiding movements in multiple planes of motion. Therefore, the focus of the participant may be to concentrate on the stabilization of the shoulder joint instead of the rigidness of the spinal column, perhaps causing a decrease in EMG activation in the RA. However, with the suspension device, there are lesser degrees of freedom with which the upper body of the participant can move, thereby causing increased attention on RA contraction to resist spinal movement.
With the EO, the increasing degrees of freedom and distance of the center of mass play a crucial role in EMG activity. Between the instability devices, the FB displayed the highest EO output, whereas EB had the lowest. The FB and EB were significantly different, and the magnitude of the difference was considered large (Cohen's d = 0.70) (3). This can once again be explained by the increased distance between the participant and the labile surface. However, the results are reversed when it comes to the suspension trainer, such that the ET elicits a higher activation than when the feet are placed in the suspension handles. It may appear that the suspension trainer provides a minimal decrease in stabilization when the feet are placed on the labile surface as compared with the stability ball. Therefore, it may seem that as the degrees of freedom increase with an instability device, EO activity may increase to resist additional spinal twisting and torsion.
In terms of the LSES, significant differences also exist between the 2 instability devices examined. The plank with the forearms placed on the labile surfaces (ET and EB) produced similar EMG activation in the lumbar region. Yet, the 2 planks where the feet came in contact with the instability devices produced decreased, significant (FT only), muscular activity. It can be speculated that limb positioning plays an important role in LSES activity while performing the plank. However, it has been stated previously that with an increase in lower lumbar muscular activation, an increase in spinal tension and compression may occur. Consequently, planks that increase LSES activity may be considered an advanced movement and perhaps avoided by those with lower-back health injuries.
This study only used the TRX and Swiss ball during the data collection process. They were chosen because of their immense popularity within the fitness and exercise culture. Therefore, the findings should not be extrapolated to other commercial instability devices, such as the BOSU ball and dyna discs. Furthermore, the 2 instability devices differ in terms of body positioning. Performing a plank with a suspension device places the body in a more horizontal posture compared with the Swiss ball. The participants of the study did not deviate from the relative “normal” body posture that each device requires. Future research is warranted to cross-analyze the instability devices with an equalized horizontal posture to determine if a deviation of the required body position affects EMG activity. Additional research may also include measurement of core musculature activity during other isolated abdominal exercises using additional instability equipment.
Practical Applications
Based on EMG activations alone, our study demonstrated increased RA, EO, and LSES activation when planks are performed on 2 types of instability devices (e.g., Swiss ball and TRX) compared with the traditional plank. Although the traditional plank can provide an adequate stimulus to increase anterior trunk muscular strength and endurance (14,15), the instability devices may serve as a means of progression for those wanting an increased abdominal challenge. Practitioners should take note that individuals with poor abdominal strength and endurance may benefit from the incorporation of planks performed with instability devices (6). However, those individuals with previous lower-back injury or localized weaknesses of the spinal erectors should use caution with the addition of such devices into a training program.
Acknowledgments
The authors thank Tim Hewitt for his assistance in data collection. The authors also thank TRX (Fitness Anywhere, LLC) and POWER SYSTEMS, Inc., for supplying the TRX Suspension Trainer systems for this investigation. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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